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Mid-crustal electrical conductors and their correlations to seismicity and deformation at Itoigawa-Shizuoka Tectonic Line, Central Japan

Abstract

An active fault segment at the northern Itoigawa-Shizuoka tectonic line (ISTL), Central Japan, which will potentially cause M8-class intraplate earthquake, was imaged by wide-band magnetotellurics. Three parallel profiles across ISTL revealed along-strike variation of the resistivity structure. Three resistivity models commonly showed the thickening conductors in the upper crust to east of ISTL which imply the heavily folded Miocene sediments with maximum thickness of several kilometers. Thus the upper crustal structure seems two-dimensional throughout the segment. We found mid-crustal conductors, top of which correlate well with the cutoff depth of seismicity. The seismicity clusters mainly in the resistive crust that is underlain by the mid-crustal conductors. This implies the local distribution of fluids below the brittle-ductile boundary and suggests that the fluid migration into resistive zone is triggering earthquakes. However, the distribution of these mid-crustal conductors is not consistent with the strike of ISTL, but rather it is better correlated with the negative dilatation anomaly inferred from GPS. This suggests the weakening of the crust by the existence of fluids.

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Correspondence to Yasuo Ogawa.

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Ogawa, Y., Honkura, Y. Mid-crustal electrical conductors and their correlations to seismicity and deformation at Itoigawa-Shizuoka Tectonic Line, Central Japan. Earth Planet Sp 56, 1285–1291 (2004). https://doi.org/10.1186/BF03353352

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Key words

  • Itoigawa-Shizuoka Tectonic Line
  • magnetotellurics
  • deformation
  • fluids